To polymers such as rubbers are generally added antioxidants or the like to impart properties such as ozone resistance (see Patent Literature 1). For example, the tread rubber of a tire incorporates a phenylenediamine antioxidant (6C) or a ketone/amine condensate antioxidant (RD) in order to prevent brown discoloration due to blended chemicals as much as possible while maintaining oxidation resistance, ozone resistance, flex crack growth resistance, and UV resistance. Further, the tread rubber also incorporates wax in order to suppress static crack growth.
Tires are known to deteriorate due to radicals generated in rubber when exposed to ozone, ultraviolet light, or oxygen. It is considered that if an antioxidant can rapidly trap such radicals and convert them into harmless ones, reactions of radicals to double bonds in rubber and breakage of polymer chains can be prevented, and generation and growth of cracks can therefore be suppressed. For this reason, conventionally, a large amount of antioxidant is added to suppress generation and growth of cracks particularly resulting from ozone deterioration.
However, although tread rubbers and sidewall rubbers of tires, which contain a phenylenediamine antioxidant, are excellent in ozone resistance and flex crack growth resistance, the phenylenediamine antioxidant can easily bloom to the tire surface, and the tires have poor heat stability such as being volatile and also have the problem of brown discoloration due to chemicals. On the other hand, if the amount of phenylenediamine antioxidant is reduced to avoid brown discoloration, then rubbers are remarkably deteriorated, which can easily cause breakage phenomena such as generation and growth of cracks and rubber chipping. Particularly, tires mounted on frequently driven vehicles have less problem of brown discoloration because a newly exposed tread rubber surface is formed due to tire wear. On the other hand, tires mounted on less frequently driven vehicles and tread groove bottoms on which no wear is caused can turn brown to cause a problem in appearance.
Moreover, on sidewalls, the same rubber surface is always exposed, and the history of deterioration including brown discoloration is accumulated on the surface. Thus, such brown discoloration is noticeable in appearance when the tires are mounted. Therefore, sidewalls more often have the problem of brown discoloration than treads. When sidewalls turn brown, users may post-apply commercial glossing wax or the like to the tires. Some kinds of wax may promote transfer of an antioxidant to the rubber surface and volatilization of the antioxidant to impair ozone resistance and thereby promote cracking of sidewalls. As described above, the measures using only known antioxidants have problems in suppressing deterioration of rubber compositions due to ozone and the like.